How is relative humidity defined?

The definition of relative humidity is based on the concept of how much moisture the air can hold compared to what it is currently holding. The correct answer emphasizes that relative humidity is a ratio that compares the current amount of water vapor present in the air to the maximum amount of water vapor that the air could hold at a specific temperature. This measurement is expressed as a percentage, indicating how close the air is to being fully saturated with moisture.

While total moisture content of air and the amount of water vapor in the air provide relevant information about humidity, they do not encapsulate the concept of relative humidity, which specifically considers the capacity of the air to hold moisture at the given temperature. The condition of air at the saturation point relates to 100% relative humidity but does not define the term itself, as relative humidity can vary significantly below that point. Thus, the correct choice accurately captures the essence of relative humidity as a measure of how much moisture the air currently holds relative to its capacity.